In order to provide optical recording media which enable to be recorded and reproduced over a blue laser wavelength range or shorter wavelength range, blue laser beams enabling ultra-high density recording have been rapidly developed along with development of a recordable optical recording media to which such blue laser beams can be used.
The inventors of the present invention propose the availability of an optical recording media having a recording layer which primarily contains a metal oxide or a half metal oxide, particularly bismuth oxide (Patent Literatures 1 to 4) as write-once-read-many optical recording media enabling to be recorded at high density over a wavelength range for blue laser. In Patent Literature 5 and the like, the applicant of the present invention proposes that a write-once-read-many optical recording medium having a Bi—B—O (oxygen) containing-recording layer exhibits still better properties (hereinafter, a technology relating to these application are referred to as “applicant's conventional technologies”).
In the applicant's conventional technologies, the write-once-read-many optical recording media have been confirmed to have excellent recording and reproducing property.
Besides the above-described applicant's conventional technologies, Patent Literature 6 discloses an optical recording medium having a recording layer which primarily contains bismuth oxide, but does not contain B.
Optical discs have been developed on high density recording and speeding up of recording. In conventional DVD high density has been achieved by providing two layers, and optical discs recordable at 16× are available in terms of speeding up of recording. Optical discs using a blue laser diode may also be developed for high velocity recording. Thus, optical recording media for high velocity recording are beginning to be developed.
However, high velocity recording has not been considered in the applicant's conventional technologies. Patent Literature 6 has an object to improve recording and reproducing property and reliability (reproduction stability and storage stability), and a material in which various elements X are added to bismuth oxide is considered. However, in all Examples one element X is added to the bismuth oxide and does not contain B, and specific examples of materials in which two or more of the elements X are added to the bismuth oxide are not disclosed.
The sputtering method has been widely known as one of the vapor-phase deposition techniques for producing a thin layer. The method is also used in an industrial thin layer production. In the sputtering method, a target material which has the same component as a layer to be formed is prepared. Normally, argon gas ion which is generated by means of glow discharge in the target material is then allowed to collide with this target material with the result that the constituent atoms of the target material are hammered out, and atoms are accumulated on the substrate thereby forming a layer. In particular, since oxides generally have high melting points, a method such as vapor deposition is not preferably used and thus a radio frequency sputtering that applies a high frequency is often used. The sputtering method has achieved satisfactory results in the production process and is advantageous in a point of through-put. However, in the case where a layer made of a material containing two or more elements is deposited, the resultant layer often has a different composition from its sputtering target, thus the composition of the target has to be considered. Furthermore, the structure and the quality of the layer often differ depending on the form of the compound making up the target; thus, it is also necessary to consider the form of the compound.
Furthermore, it is also required to further improve a layer deposition rate from the viewpoint of the production cost. For improvement of the layer deposition rate, it is necessary to introduce larger power. In this case as well, it is necessary to improve the target strength so as to avoid breaking of the target.
For example, Patent Literature 7 discloses a Bi2O3-based glass-like target containing B2O3. However, the glass-like target in Patent Literature 7 inevitably contains SiO2 and is different in components from the target of the present invention.
Optical recording requires high density recording as well as high linear velocity recording to improve information transfer speed. High linear velocity recording needs higher recording sensitivity than low linear velocity recording. However, the applicant's conventional technologies have not been limited on recording linear velocity, and may not be suitable for high linear velocity recording.
The form and structure of a compound and impurities making up a sputtering target for a recording layer affects a composition and crystal structure of the recording layer to be formed. Therefore, the compounds making up the sputtering target is made suitable for properties of a desired recording layer.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2003-48375
Patent Literature 2: JP-A No. 2005-161831
Patent Literature 3: JP-A No. 2005-108396
Patent Literature 4: JP-A No. 2006-116948
Patent Literature 5: JP-A No. 2006-247897
Patent Literature 6: Japanese Patent (JP-B) No. 3802040
Patent Literature 7: JP-A No. 2005-264206